Question

Hydrogen is collected by downward displacement of water and not of air, even though it is lighter than air. Explain why?

Answer 1

Hint: Hydrogen is the most abundant and highly flammable element found in the universe and is insoluble in water. This method of collection is used for those gases which are insoluble.

Complete Step to Step solution:
In the laboratory, hydrogen is produced by action of $ \mathop H\nolimits_2 S\mathop O\nolimits_4 \left( {dil.} \right) $ on granulated zinc. $ Zn + \mathop H\nolimits_2 S\mathop O\nolimits_4 \left( {dil.} \right) \to ZnS\mathop O\nolimits_4 + \mathop H\nolimits_2 $
It should be kept in mind that, most of the gases that are produced in the laboratory are usually collected by a technique called water displacement due to the fact that they are insoluble in water.
In this technique, firstly a bottle is filled and placed upside-down in a tray of water. When the reaction flask in which the above written reaction takes place is fitted with rubber tubing and is kept under the bottle of water then it is observed that the gas i.e. hydrogen gas which is evolved in the reaction flask exit through rubber tubing and displaces the water kept in bottle. After some time it is observed that the bottle is now filled with hydrogen gas.

Although hydrogen is lighter than air but then also it cannot be collected by downward displacement of air. This is due to the fact that hydrogen gets oxidized easily in presence of air. This means that hydrogen can easily mix with air and thus, cannot be collected by this method.
Therefore, from above we get to know that although hydrogen is lighter than air but it is collected by downward displacement of water not of air.

Note: It should be kept in mind that hydrogen mixes with air but does not dissolve in water. So, it is collected by displacement of water.